Medium+Access+Control

Medium Access Control Protocols

Overview

  • Focused on communication networks including multiple access communications.

Multiple Access Communications

  • Shared media basis for broadcast networks.

  • Effective for inexpensive media like radio, copper, or coaxial cable.

  • Multiple users communicate by broadcasting into the medium.

  • Key issue: How to efficiently share the medium.

MAC Sublayer

  • Data link layer divided into two sublayers:

    • LLC (Logical Link Control)

    • MAC (Medium Access Control)

Medium Sharing Techniques

  1. Static Channelization

  2. Dynamic Medium Access Control

  3. Scheduling Techniques:

    • Random access, polling, token ring, and wireless LAN.

    • Examples: Aloha and Ethernet.

Random Access and Token Passing

  • Random Access: Transmit when ready; handle collisions (retransmission required).

  • Token-Passing: Stations transmit when they hold the token; reduces collision risks.

CSMA Protocols

  • CSMA (Carrier-Sensing Multiple Access):

    • Stations sense the channel before transmitting.

    • Variants include 1-persistent, non-persistent, and p-persistent CSMA to balance delay and efficiency.

  • CSMA/CD: Monitors for collisions and aborts transmissions, reducing wasted time.

ALOHA Protocol

  • Basic approach where a station transmits whenever it has data.

  • Collisions handled by random backoff time.

  • Two variations: ALOHA and Slotted ALOHA.

Scheduling Methods

  • Reservation Systems: Centralized and decentralized systems for frame transmission with the option for single or multiple frames.

  • Polling Systems: Centralized or distributed polling to manage how stations transmit data.

Token Ring and Its Efficiency

  • Token-Passing for stable throughput in ring networks.

  • Single vs. multi-token operation influences throughput; strategies vary based on traffic load.

MAC Performance Metrics

  • Frame transfer delay and throughput are key performance metrics.

  • Efficiency depends on the load, with maximum efficiency typically less than 100% because of collisions and delays.

Comparison of MAC Approaches

  • ALOHA/Slotted ALOHA: Good for low traffic but can hit variable delays.

  • CSMA/CD: Effective for bursty users with better efficiency at low loads.

  • Reservation: Provides QoS and handles bursts efficiently.

  • Polling: Fairness through regular access but can suffer delays at high traffic.